Process for chromizing ferrous metal objects



N m 0 R W T R S R Hm E P s A J L G M w I l I I I I I I I l I I I l l l lI l I I I l i I I O T D v M55376 558 $2135. 5 m d 532$ 923:6 It; $2.528120.25%: mmwzamozz ESE T l1 \imwxmmozb W I m I I L1 a- 1 k MED. i 2954mmu n n mo z-5 Oct. 6, 1964 PROCESS FOR CHROMIZING FERROUS METAL OBJECTSATTORNEY United States Patent 3,152,007 PROCESS FDR CEiOMiZlNG FERROUhdETAL OBJECTS Torn S. Perrin, 'iainesville, and Douglas H. Strong,

Willoughby, Ohio, assignors to Diamond Alkali Company, Cleveland, Ohio,a corporation of Delaware Filed Apr. 13, 1961, Ser. No. 102,652 7Claims. (Ci. 117-107.2)

This invention relate to a process for protecting ferrous metal surfacesfrom corrosion and oxidation by forming a chromized surface thereon.

In the usual chormizing process, the object to be treated is broughtinto contact with vapors of a chromium chloride or with chromium formedby the reduction of these vapors, at a temperature of about 1500-1900 F.The chromium chloride vapors undergo an exchange reaction with the ironon the surface of the object, with the formation of metallic chromiumand its diffusion into the surface. In addition, the elemental chromiumformed by the reduction of the chromous chloride itself diffuses intothe surface of the object being treated, the result being the formationof an alloy of high chromium content, resistant to corrosion andoxidation, on the surface of the object being treated.

Previously described chromizing procedures have generally fallen intoone of two classes. The first of these employs elemental chromium whichis converted to chroznous chloride by the action of hydrochloric acid ora carrier material such as ammonium chloride. This method suffers fromthe disadvantage that at the temperatures normally employed the vaporpressure of the reacting species, chromous chloride, is very low, about0.0005 atmosphere. At this pressure, the formation of a chrornized layerof suitable thickness requires a long period of time.

In the second method, the chromium source is either chromous chloride ora mixture of chromium chlorides. At the reaction temperature thechromous chloride is vaporized and transmitted, either by a poroustransmitting medium or by a gas, usually hydrogen, to the surface of themetal to be coated. The chromic chloride, if any, in the mixture isreduced by hydrogen to chromous chloride before vaporization and, assuch, is transmitted to the metal surface. This method suffers from thesame disadvantage as the previous one; namely, that the vapor pressureof chromous chloride at the temperatures employed for chromizing isquite low (0.0490 atmosphere at 1827 F.) and that the process is,therefore, very timeconsurning. Also, chromous chloride is a ratherunstable compound which is difiicult and expensive to prepare and store.

It is the object of the present invention to provide an economicalprocess for producing a chromized surface on a ferrous metal object. Afurther object of the present invention is to reduce the time nowrequired for chromizing ferrous articles. These and other objects willbecome apparent to those skilled in the art upon reading the descriptionof the process which follows.

In the drawing: The FIGURE is a diagrammatic illustration of anexperimental apparatus used in this invention.

The method of this invention employs a chromic chloride, CrCl as achromium source in the chromizing process. Chromic chloride has theadvantage of being less expensive and easier to obtain and handle thanchromous chloride and, in addition, has other advantages which are morefully described hereinbelow.

3,152,0h7 Patented Get. 6., 1964 "ice. A

The chemical reactions simultaneously involved when chromizing with thechromium chloride and hydrogen are as follows:

(1) CrCl +ll ?2CrCl 21-101 (2) CrCl +H Cr+2HCl (3) 2Fe(metal surface)+CICl e FeCr(surface case) +FeCl (4) FCIQ+HQZFB+ 21-101 (5) Fe (metalsurface) Cr FeCr (surface case) A protective case formed on the metalsurface is brought about when chromium difiuses into the metal surfaceas a result of Reaction 3 and when chromium is deposited on the metalsurface as a result of Reaction 5. It has been estimated that about halfof the case formed on the metal surface comes from Reaction 3 and halffrom Reaction 5. The thickness of the protective case formed by thisprocess depends upon: (a) the prevailing temperature while the metalsurface is exposed to the chromium chloride and hydrogen; (b) the vaporpressure of the chromium chloride; (0) the ratio of H and HCl duringchromizing; (d) the time of exposure of the metal surface at chromizingconditions.

The temperature is important because it determines the vapor pressureor" the chromous chloride and also the rate of didusion of chromium intothe metal surface. Normally, temperatures of 13002000 F. are used.

The vapor pressure of chromous chloride is determined by the mode ofconducting the chromizing process. The vapor pressure of chromicchloride is much higher than that of chromous chloride as shownhereinbelow; therefore, if hydrogen is not mixed with CrCl until it isvaporized, the CrCl vapors can be reduced to give CrCl at the same vaporpressure. This is the process which has been used in the presentinvention.

CrOI atm. GrCl atm,

The amount of hydrogen used, or the hydrogen flow, dete mines the HChl-lratio, and this ratio determines the extent to which the equilibriumReactions 1-4 take place. At the temperatures used in the chromizingprocess, Reaction 1 goes essentially to completion. The HCl thus formedsuppresses Reactions 2 and 4, and, therefore indirectly suppressesReaction 3 by decreasing the rate of removal of FeCl from the system byReaction 4. A balance must, therefore, be maintained between a rapidhydrogen flow, which would sweep the HCl out of the system and promoteReactions 2 and 4 but would also sweep away appreciable quantities ofCrCl vapor, and a slow flow, which does not sweep out the CrCl but alsoleaves HCl in the system and retards Reactions 2 and 4. it has beenfound that a replacement rate per hour of about 0.5-1.0 mole of hydrogenper mole of chromic chloride used is optimum.

The experimental apparatus used in this invention and illustrateddiagrammatically in the drawing consists of a furnace through which runsa horizontal tube with an inlet for gas at one end and an outlet at theother end. The furnace should be suitable for heating the chromizingchamber to a maximum temperature of about 2000" F. In the approximatecenter of the tube, in the heating zone, is placed a vessel containingchromic chloride, fitted with a perforated cover allowing for diffusionof the chromic chloride vapors but excluding most of the hydrogen toprevent reduction of the solid CrCl Samples to be chromized aresuspended above this vessel. It is also desirable to provide a means forthorough mixing of the hydrogen and chromic chloride vapors.

The system is swept with an inert gas, usually dry nitrogen, helium orargon, while the temperature is raised to about BUT-1550 1 preferably1400-l500 F. At this temperature, the atmosphere is changed to dryhydrogen, adding about 0.51.0 mole of hydrogen per mole of CrCl asquickly as possible and thereafter replacing the atmosphere withhydrogen at a rate of approximately 0.5-1.0 mole of hydrogen per mole ofCrCl per hour. The system is maintained at chromizing temperature,

usually approximately 15002000 F., for 1-2 hours for about a 2 milcoating. The system is then cooled as quickly as possible to about 1000F. while continuing the hydrogen flow. At this temperature the hydrogenis replaced by the inert gas and the temperature is cooled to about 200F.

The following example is given in order that those skilled in the artmay more completely understand the invention and the means for carryingthe same into effect.

Seventy-five grams of chromic chloride is placed in a vessel with aperforated cover and placed in the heating zone below the samples to bechromized. In an argon atmosphere, the vessel is heated to 1500 F. Theatmosphere is converted to hydrogen at this point. The

rate of addition or" hydrogen is controlled by igniting the exhaustgases and adjusting the flow so that the flame is approximately %-inchlong. (Alternatively, a fiowmeter may be used to measure the hydrogenflow.) Rapid hydrogen chloride evolution begins with the introduction ofhydrogen. perature reaches 1800 F. It is held at approximately 1860 F.for 2 hours. The hydrogen flow is increased slightly and the vessel iscooled to 950 F. over a minute period. At this temperature the evolutionof hydrogen stops and the atmosphere is reconverted to argon. Aftercooling to 200 F., the samples are removed. After sectioning, polishingand etching with 2:1 nitric acid solution, the samples are shown to havea chromized coating varying in thickness from 0.0020.003 inch. Thiscoating does not react with boiling 20% ntric acid.

It is to be understood that, although the invention has been describedwith specific reference to particular em bodiments thereof, it is not tobe so limited, since changes and alterations therein may be made whichare within- After 2 hours and 40 minutes, the fem the full intendedscope of this invention as defined by the appended claims.

What is claimed is:

l. A process for chromizing ferrous metal objects comprising the stepsof heating the object to be chromized and chromic chloride in an inertatmosphere to a temperature such that the vapor pressure of chromicchloride is at least about 0.015 atmosphere, said chromic chloride beingcontained in a vessel with a perforated cover; passing hydrogen throughthis system to effect reduction of a vaporized chromic chloride to formgaseous chromous chloride and chromium at approximately the same vaporpressure; and contacting the surface of the object to be chromized withsaid gaseous chromous chloride and chromium.

2. A process for chromizing ferrous metal objects comprising the stepsof heating the object to be chromized and chromic chloride to atemperature of about 1300- 1550 F. in an inert atmosphere, said chromicchloride being contained in a vessel with a perforated cover; passinghydrogen through the system at a temperature of about 15002000 F. toeffect reduction of the vaporized chromic chloride to form gaseouschromous chloride and chromium at approximately the vapor pressure ofthe chromic chloride; and contacting the surface of the object to bechromized with said gaseous chromous chloride and chromium.

3. A process for chromizing ferrous metal objects comprising the stepsof heating the object to be chromized and chromic chloride to about1400-1500 F. in an inert atmosphere, said chromic chloride beingcontained in a vessel with a perforated cover; passing hydrogen throughthe system at a temperature of about 1800 1900" F. to effect reductionof the vaporized chromic chloride to form gaseous chromous chloride andchromium at approximately the vapor pressure of the chromic chloride;and contacting the surface of the object to be chromized with saidgaseous chromous chloride and chromium.

4. The process of claim 3 wherein the inert atmosphere is nitrogen.

5. The process of claim 3 wherein the inert atmosphere is argon.

. 6. The process of claim 3 wherein the inert atmosphere is helium.

7. The process of claim 3 wherein the hydrogen flow rate approximately0.5 mole of hydrogen per mole of CrCl per hour.

References Qited in the file of this patent Powell: Vapor Plating(1955), John Wiley (N.Y.), pp. 47 and 48 relied on.

' Insausti: Chromizing of Steel, translated from Instituto del Hierro ydel Acero, vol. 9, March 1956, No. 44, pp. 250257 (p. 5 of translationrelied on).

1. A PROCESS FOR CHROMIZING FERROUS METAL OBJECTS COMPRISING THE STEPSOF HEATING THE OBJECT TO BE CHROMIZED AND CHROMIC CHLORIDE IN AN INERTATMOSPHERE TO A TEMPERATURE SUCH THAT THE VAPOR PRESSURE OF CHROMICCHLORIDE IS AT LEAST ABOUT 0.015 ATMOSPHERE, SAID CHROMIC CHLORIDE BEINGCONTAINED IN A VESSEL WITH A PERFORATED COVER; PASSING HYDROGEN THROUGHTHIS SYSTEM TO EFFECT REDUCTION OF A VAPORIZED CHROMIC CHLORIDE TO FORMGASEOUS CHROMOUS CHLORIDE AND CHROMIUM AT APPROXIMATELY THE SAME VAPORPRESSURE; AND CONTACTING THE SURFACE OF THE OBJECT TO BE CHROMIZED WITHSAID GASEOUS CHROMOUS CHLORIDE AND CHROMIUM.